Substrate strip for use in packaging semiconductor chips and method for making the substrate strip

ABSTRACT

A flexible substrate strip comprises a plurality of substrate units adapted for mounting semiconductor chips. The surface of the flexible substrate strip is provided with a plurality of degating regions at locations such that the edges of mold runners and gates of a mold used to encapsulate the semiconductor chips in encapsulant material fit entirely within the degating regions when the substrate strip is placed in the mold during encapsulation of the semiconductor chips. The present invention is characterized in that each degating region has a buffer region at a location corresponding to the gate of the mold during encapsulation. The degating regions have a degating region material formed thereon with the buffer regions not coated with the degating region material. The adhesive force between the encapsulant material and the degating region material is less than the adhesive force between the encapsulant material and the substrate. The present invention also provides a method of making the flexible substrate strip.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a flexible substrate strip for use informing a plurality of substrate-based semiconductor chip packageswherein the excess encapsulant can be removed without damaging thepackaged electronic device after encapsulating the semiconductor chip.

[0003] 2. Description of the Related Art

[0004] As the need for lighter and more complicated semiconductordevices becomes greater day by day, the semiconductor chips have becomemore and more complex thereby requiring more electrical connections.Therefore, the ball grid array (BGA) has been developed by thesemiconductor chip packaging industry to meet these needs.

[0005] A typical BGA package generally includes a chip mounted to theupper surface of a substrate i.e. a printed circuit board. Bonding padson the active surface of the chip are connected to electricallyconductive traces formed on the upper surface of the substrate bybonding wires. The lower surface of the substrate is provided with aplurality of solder pads electrically connected to the electricallyconductive traces. Each solder pad is mounted with a solder ball formaking external electrical connection. A package body is formed toenclose the chip, the bond wires and a portion of the upper surface ofthe substrate including most of the electrically conductive traces. Thepackage body is typically formed by a transfer molding process.

[0006]FIG. 1 is a top plan view of a conventional transfer moldingequipment with flexible substrate strips attached. As shown, the moldingequipment includes three transfer pots 201 for accommodatingencapsulant. Each transfer pot 201 of the molding equipment has atransfer rams 204 positioned therein and is connected to four gates 208through a mold runner 206. Each gate 208 is connected to a cavity 210.Two flexible substrate strips 230 are placed in the molding equipment ina manner that each substrate unit of the strips 230 adapted for mountinga semiconductor chip is corresponding to each cavity 210 of the moldingequipment.

[0007] After the encapsulant is loaded in the transfer pots 201 and theflexible substrate strips 230 are fixed and clamped by the moldingequipment, the transfer ram 204 is moved downwardly to compress theencapsulant. The molding equipment and encapsulant are pre-heated sothat when the transfer ram 204 compresses the encapsulant, the liquefiedencapsulant is forced through the mold runners 206 and gates 208 to fillthe cavities 210 and thereby encapsulating the semiconductor chips (notshown) mounted on the flexible substrate strips 230. After theencapsulant fills the cavities 210, the transfer ram 204 stands stillfor a predetermined time until the encapsulant cures. Then the transferram 204 is withdrawn, the molding equipment is opened, and the moldedproducts is removed from the molding equipment. Extra parts such asrunners and gates are removed from the molded products, and then themolded products are cut into individual units, whereby the semiconductorchip packages are completed.

[0008] However, one shortcoming of the above process is apparent.Specifically, the encapsulant not only fills the cavities 210 but alsofills the gates 208, the mold runners 206 and the transfer pots 201.Therefore, when the encapsulant is cured, the cured encapsulant not onlycovers the semiconductor chips, but also extends along the surface ofthe flexible substrate strip 230, where the gates 208 and the moldrunners 206 are located, and into the pots 201. This excess curedencapsulant is often referred to as the “runner” and must be removedbefore the molded products are singulated. Accordingly, the gate 208 isgenerally made smaller in cross-sectional area than the mold runner 206in order to assist in the “degating” process, i.e., the removal of theexcess encapsulant. However, the encapsulant tends to adhere to thesurface of the substrate, so the removal of the excess encapsulant islikely to twist the flexible substrate strip and causes damage to thesurface thereof.

[0009] Therefore, it is desirable to provide degating regions 220 on theflexible substrate strip 230 such that the edges of mold runners 206 andgates 208 fit entirely within the degating regions 220 duringencapsulation of the chips. Typically, a degating region material suchas gold is formed on the degating regions wherein the adhesive forcebetween the encapsulant and the degating region material is less thanthe adhesive force between the encapsulant and the substrate whereby theexcess encapsulant can be removed without damaging the flexiblesubstrate strip.

[0010]FIG. 2 is a partial side view of a packaged product 240 withto-be-removed excess encapsulant. As shown in FIG. 2, the gate 208 (forsimplicity, the mold runner 206 and the transfer pot 201 are not shownin FIG. 2) is still linked with the packaged product 240. Afterdegating, it is desirable to have a substantially regular breakingsurface as shown in FIG. 3. However, there is nearly no resistance forceduring the removal of the excess encapsulant from the degating region,hence the breaking force is so violent as to generate the defect resultsas shown in FIG. 4 and FIG. 5. FIG. 4 shows a bulge 242 protruding fromthe outline of the packaged product. The bulge 242 will cause damage tothe punch tool during the singulation process. FIG. 5 shows a concave244 formed in the packaged product. If the damage caused by theformation of the concave 244 is severe enough, it may harm electricallyconductive traces on the flexible substrate strip. Even if there is nodirect damage to the substrate, the concave 244 resulting from thedegating process may still weaken the seal between the molded body andthe upper surface of the substrate, thereby increasing the chances ofmoisture penetration in the packaged product.

SUMMARY OF THE INVENTION

[0011] It is a primary object of the present invention to provide aflexible substrate strip for use in forming a plurality of semiconductorchip packages wherein each degating region of the flexible substratestrip is provided with a buffer region formed therein for alleviatingthe breaking force during degating process thereby obtaining asubstantially regular breaking surface.

[0012] A flexible substrate strip in accordance with the presentinvention comprises a plurality of substrate units adapted for mountingsemiconductor chips. The flexible substrate strip is provided with aplurality of degating regions in such manner that the edges of moldrunners and gates of a mold used to encapsulate the semiconductor chipsin encapsulant material fit entirely within the degating regions whenthe substrate strip is placed in the mold during encapsulation of thesemiconductor chips. The present invention is characterized in that eachdegating region has a buffer region at a location corresponding to thegate of the mold during encapsulation. The degating regions have adegating region material formed thereon while the buffer regions are notcoated with the degating region material. The adhesive force between theencapsulant material and the degating region material is less than theadhesive force between the encapsulant material and the substrate strip.

[0013] Since the buffer regions have no degating region material formedthereon, the surface of the buffer regions has the same characteristicsas the substrate strip. Accordingly, the better adhesion of theencapsulant to the surface of the buffer regions helps to alleviate theviolent breaking force during the degating process thereby rendering thebreaking surface formed on the packaged product more regular.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other objects, advantages, and novel features of the inventionwill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

[0015]FIG. 1 is a top plan view of conventional flexible substratestrips placed in a conventional molding equipment;

[0016]FIG. 2 is a side view of a packaged product with to-be-removedexcess encapsulant;

[0017]FIG. 3-5 illustrates three different results of the packagedproduct of FIG. 2 after removing the excess encapsulant;

[0018]FIG. 6 is a top plan view of a portion of a flexible substratestrip according to a first embodiment of the present invention placed ina conventional molding equipment;

[0019]FIG. 7 is a detailed plan view of a portion of the flexiblesubstrate strip according to the first embodiment of the presentinvention placed in a conventional molding equipment;

[0020]FIG. 8 a detailed plan view of a portion of a flexible substratestrip according to a second embodiment of the present invention placedin a conventional molding equipment; and

[0021]FIG. 9 a detailed plan view of a portion of a flexible substratestrip according to a third embodiment of the present invention placed ina conventional molding equipment

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022]FIG. 6 is a plan view of a portion of a flexible substrate strip300 according to a first embodiment of the present invention placed in aconventional molding equipment. The flexible substrate strip 300comprises a plurality of substrate units 310. Each substrate unit 310 isadapted for mounting a semiconductor chip. Though only four substrateunits 310 of the flexible substrate strip 300 are shown in FIG. 6, aflexible substrate strip for use with the invention can include anynumbers of substrate units that is compatible with the manufacturingequipment, e. g., mold, being used.

[0023] Each substrate unit 310 is provided with a plurality of leads 310a and connection pads 310 b. Each lead 310 a has one end adapted forelectrical connecting to a semiconductor chip (not shown) and the otherend connected to the corresponding connection pad 310 b. The connectionpads 310 b are disposed corresponding to through-holes (not shown)formed in the substrate strip. Each connection pad 310 b has at least aportion exposed within corresponding through-hole for mounting a solderball (not shown).

[0024] The flexible substrate strip 300 is provided with a plurality ofdegating regions 320 (only one is shown in FIG. 6) at locations suchthat the edges of mold runners 322 and gates 324 of a mold used toencapsulate the semiconductor chips in encapsulant material fit entirelywithin the degating regions 320 during encapsulation of thesemiconductor chips. Each degating region 320 has a buffer region 330 aat a location corresponding to the gate 324 of the mold duringencapsulation.

[0025]FIG. 7 is a detailed plan view of a portion of the flexiblesubstrate strip 300 placed in a conventional molding equipment forfurther illustrating the buffer region 330 a. Compound line 350 is theoutline of package body enclosed the semiconductor chip. After degatingprocess, the molded product is cut into individual units along the punchline 340 thereby obtaining the finished semiconductor chip packages. Adegating region material is formed on the degating region 320 with thebuffer region 330 a not coated with the degating region material in amanner that a rectangular gap appears at a location corresponding to thebuffer region 330 a. Preferably, the width of the rectangular gap is0.13 mm. The adhesive force between the encapsulant material and thedegating region material is approximately 10% of the adhesive forcebetween the encapsulant material and the substrate strip. Therefore, thebetter adhesion of the encapsulant to the surface of the buffer region330 a helps to alleviate the violent breaking force during degatingprocess thereby rendering the breaking surface formed on the packagedproduct more regular.

[0026]FIG. 8 is a detailed plan view of a portion of a flexiblesubstrate strip according to a second embodiment of the presentinvention placed in a conventional molding equipment. The flexiblesubstrate strip according to the second, embodiment of the presentinvention is substantially identical to the flexible substrate strip 300of FIG. 6 with exception that the buffer region 320 b of the degatingregion 320 is at a different location. The buffer region 320 b is arectangular region entirely located within the area covered by the gate324 of the mold. The adhesive force between the encapsulant material andthe degating region material is approximately 10% of the adhesive forcebetween the encapsulant material and the substrate strip. Therefore, thebetter adhesion of the encapsulant to the surface of the buffer region330 b helps to alleviate the violent breaking force during degatingprocess thereby rendering the breaking surface formed on the packagedproduct more regular.

[0027]FIG. 9 is a detailed plan view of a portion of a flexiblesubstrate strip according to a third embodiment of the present inventionplaced in a conventional molding equipment. The flexible substrate stripaccording to the third embodiment of the present invention issubstantially identical to the flexible substrate strip 300 of FIG. 6with exception that the buffer region 320 c of the degating region 320is at a different location. The buffer region 320 c is located next tothe substrate unit 310. The adhesive force between the encapsulantmaterial and the degating region material is approximately 10% of theadhesive force between the encapsulant material and the substrate strip.Therefore, the better adhesion of the encapsulant to the surface of thebuffer region 330 b helps to alleviate the violent breaking force duringdegating process thereby rendering the breaking surface formed on thepackaged product more regular.

[0028] The method of making the flexible substrate strip for use informing a plurality of semiconductor chip packages in accordance withthe present invention is as follows:

[0029] (A) A plurality of through-holes are formed in a flexiblesubstrate by conventional punching technique. The flexible substrate maybe a resin film. Preferably, the flexible substrate is made ofpolyimide.

[0030] (B) An electrically conductive metal layer such as copper foil islaminated on the flexible substrate by conventional method such asthermocompression.

[0031] (C) The metal layer is etched to form a plurality of leads 310 a,a plurality of connection pads 310 b, and a metal covering defining aplurality of degating regions 320 wherein each buffer region 330 a hasno metal covering formed thereon. The step (C) includes forming aphotoresist layer over the metal layer, pattern transferring byphotolithography, removing the unprotected portion of the metal layer toform corresponding leads 310 a, connection pads 310 b, and the metalcovering on degating regions 320 by etching, and finally removing theremaining photoresist layer.

[0032] Each lead 310 a has one end adapted for electrical connecting toa semiconductor chip and the other end connected to the correspondingconnection pad. The connection pads are disposed corresponding to thethrough-holes. Since the through-holes are covered by the connectionpads, they are not shown in FIG. 6. Further, the backside surface ofeach connection pad 310 b has at least a portion exposed withincorresponding through-hole for mounting a solder ball (not shown).

[0033] The degating region 320 is located such that the edge of moldrunner 322 and gate 324 of the mold fit entirely within the degatingregion 320. The buffer region 330 a is at a location corresponding tothe gate of the mold during encapsulation.

[0034] (D) A degating region material is formed on the leads 310 a, theconnection pads 310 b, and the metal covering on the degating regions320. The degating region material can be electroplated usingconventional techniques so that the degating region material is formedonly on the metal layer. Since the degating region material is alsoformed on the leads 310 a adapted for electrical connecting to asemiconductor chip, the degating region material must be a material thatallows a good bond to the conventional bonding wire material. Further,the adhesive force between the encapsulant material and the degatingregion material should be less than the adhesive force between theencapsulant material and the substrate strip. Thus, gold is a suitabledegating region material.

[0035] According to the flexible substrate of the present invention,since the buffer regions have no degating region material formedthereon, the surface of the buffer regions has the same characteristicsas the substrate strip. Accordingly, the better adhesion of theencapsulant to the surface of the buffer regions helps to alleviate theviolent breaking force during degating process thereby rendering thebreaking surface formed on the packaged product more regular.

[0036] Further, during the degating process, runners are peeled offfirst and gates are removed later. Accordingly, the buffer regions ofthe flexible substrate strip of the present invention are disposedcorresponding to the gates of a mold thereby providing best bufferingeffect whereby the breaking surface formed on the packaged product issubstantially regular to avoid damaging the punch tool, harmingelectrically conductive traces on the flexible substrate strip, orweakening the seal between the molded body and the substrate.

[0037] Although the invention has been explained in relation to itspreferred embodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

What is claimed is:
 1. A substrate strip comprising: a plurality ofsubstrate units on a surface of the substrate strip, each substrate unitbeing adapted for mounting a semiconductor chip; a plurality of degatingregions on the surface of the substrate strip at locations such that theedges of mold runners and gates of a mold used to encapsulate thesemiconductor chips in encapsulant material fit entirely within thedegating regions when the substrate strip is placed in the mold duringencapsulation of the semiconductor chips, wherein each degating regionhas a buffer region at a location corresponding to the gate of the moldduring encapsulation; and a degating region material formed on thedegating regions with the buffer regions not coated with the degatingregion material, wherein the adhesive force between the encapsulantmaterial and the degating region material is less than the adhesiveforce between the encapsulant material and the substrate strip.
 2. Thesubstrate strip as claimed in claim 1 , wherein the substrate strip is aflexible substrate strip.
 3. The substrate strip as claimed in claim 1 ,wherein the adhesive force between the encapsulant material and thedegating region material is approximately 10% of the adhesive forcebetween the encapsulant material and the substrate strip.
 4. Thesubstrate strip as claimed in claim 1 , wherein the degating regionmaterial is gold.
 5. The substrate strip as claimed in claim 1 , whereinthe degating region material on the degating region forms gapscorresponding to the buffer regions.
 6. The substrate strip as claimedin claim 1 , wherein each buffer region is entirely located within thearea covered by the corresponding gate of the mold during encapsulationof the semiconductor chips.
 7. The substrate strip as claimed in claim 1, wherein each buffer region is next to the corresponding substrateunit.
 8. A method of making a flexible substrate strip for use informing a plurality of semiconductor chip packages comprising the stepsof: providing a flexible substrate; forming a plurality of through-holesin the flexible substrate; laminating an electrically conductive metallayer on the flexible substrate; etching the metal layer to form aplurality of leads, a plurality of connection pads, and a metal coveringdefining a plurality of degating regions wherein each lead has one endadapted for electrical connecting to a semiconductor chip and the otherend connected to the corresponding connection pad; the plurality ofconnection pads are disposed corresponding to the plurality ofthrough-holes; and the plurality of degating regions are located suchthat the edges of mold runners and gates of a mold used to encapsulatethe semiconductor chips in encapsulant material fit entirely within thedegating regions when the substrate strip is placed in the mold duringencapsulation of the semiconductor chips, wherein each degating regionhas a buffer region at a location corresponding to the gate of the moldduring encapsulation and each buffer region has no metal covering formedthereon; and forming a degating region material on the leads, connectionpads, and the metal covering on each degating region, wherein theadhesive force between the encapsulant material and the degating regionmaterial is less than the adhesive force between the encapsulantmaterial and the substrate strip.
 9. The method as claimed in claim 8 ,wherein the degating region material is gold.
 10. The method as claimedin claim 8 , wherein each connection pad has at least one portionexposed in the corresponding through-hole for mounting a solder ball.11. The method as claimed in claim 8 , wherein the degating regionmaterial is formed with gaps corresponding to the buffer regions. 12.The method as claimed in claim 8 , wherein each buffer region isentirely located within the area covered by the corresponding gate ofthe mold during encapsulation of the semiconductor chips.
 13. The methodas claimed in claim 8 , wherein the flexible substrate is a resin film.14. The method as claimed in claim 8 , wherein the flexible substrate ismade of polyimide.